The prior-art Very Long Baseline Interferometry (VLBI) system is presently being adapted to a new approach for radio astronomy involving a radio telescope placed in orbit around the earth. Typically, Very Long Baseline Interferometry (VLBI) involves simultaneous observations from widely separated radio telescopes followed by correlation of the signals received at each telescope in a central processing facility. VLBI has been an important technique in radio astronomy for over 20 years because it produces images whose angular resolution is far higher than that of any other technique.
The National Radio Astronomical Observatory (NRAO) is constructing an earth station at Green Bank, W. Va. to communicate with two orbiting satellites, namely the Russian RADIOASTRON and the VLBI Space Observatory Project (VSOP) of Japan, as illustrated in FIG. 1, to form an orbiting VLBI. The frequency allocations for the communication between an earth station 10 and the two satellites 11 and 12 are in the X and Ku bands as described in Table 1.
TABLE 1 ______________________________________ Reflector Antenna Requirements Frequency Bandwidth (GHz) (GHz) Usage Polarization ______________________________________ 7.22 0.045 RADIOASTRON LHCP Uplink 8.47 0.1 RADIOASTRON RHCP Downlink 14.2 0.1 VSOP LHCP Downlink 15.3 0.1 VSOP Uplink LHCP ______________________________________
To meet this dual-band communication requirement, the multireflector antenna at the ground station 10 shown in FIG. 2 has been proposed with a flat panel, frequency selective surface (FSS) 13, sometimes referred to in the literature as a "dichroic." This has been proposed in order to reflect Ku-band signals (13.5 to 15.5 GHz) into one of a pair of feedhorns 14 and 15 as they are received by a primary paraboloid reflector 16, reflected by a hyperboloid reflector 17 and re-reflected by the FSS panel 13 into the one Ko-band feedhorn 14. The X-band signals (7 to 9 GHz) received by the paraboloid reflector 16 and reflected by the hyperboloid reflector 17 are passed by the FSS panel 13 into the Xr-band feedhorn 15.
Alternatively, the RF reflector assembly may consist of just the primary reflector 16, typically of paraboloid configuration, having a primary focal point offset from the line of sight to a satellite. The FSS panel 13 is then interposed between the primary reflector reflector 16 and its focal point. The X-band feedhorn 15 is placed on the side of the FSS panel 13 opposite the reflector 16 to receive RF signals transmitted through the FSS panel 13 designed to be transparent to signals of a selected transmitted frequency f.sub.t in that band. The Ku-band feedhorn 14 is then placed on the same side of the FSS panel 13 as the primary reflector 16 to receive RF signals of a selected reflected frequency f.sub.r reflected by the FSS panel 13, as shown in FIG. 3 of U.S. Pat. No. 5,162,809 by the present inventor.
Because the satellite link is in circular polarization, the FSS panel 13 must have a similar response to left- and right-hand circular polarizations (LHCP and RHCP), and by extension, to transverse electric and transverse magnetic (TE and TM polarization) incident fields. In order to reduce the antenna's noise temperature, the RF insertion loss (including the ohmic loss) of the FSS panel 13 should also be minimized for an incidence angle range from normal to 40.degree.. This then requires a wide-angle FSS panel.
In the past, an array of cross-dipole patch elements were used for the FSS panel design in a subreflector of reflector antennas of Voyager (G. H. Schennum, "Frequency selective surfaces for multiple frequency antennas," Microwave Journal, Vol 16, No 5, pp. 55-57, May 1973) for reflecting the X-band waves and passing the S-band waves and the Tracking and Data Relay Satellite System (TDRSS) for diplexing the S- and Ku-band waves (V. D. Agrawal and W. A. Imbriale, "Design of a dichroic Casegrain subreflector," IEEE Trans., Vol. AP-27, No. 7, pp. 466-473, July 1979). The characteristics of the cross-dipole elements of an FSS change drastically as the incident angle changes from 0.degree. (normal) to 40.degree.. As a consequence, a large separation was required for the selected bands to minimize the RF losses for dual band applications. This is evidenced by the reflection and transmission band ratio (f.sub.r /f.sub.t) being 7:1 for a single screen FSS panel described by V. D. Agrawal and W. A. Imbriale supra, or 4:1 for a double screen FSS panel described by Schennum, supra, with cross-dipole patch elements. A better dichroic design needed to reflect Ku band signals and pass X band signals, i.e., needed to achieve smaller frequency-band separations, as required by the OVLBI application (f.sub.r /f.sub.t =14.5/8.0=1.8) is disclosed in U.S. Pat. No. 4,814,785. However, Ku band RF losses were higher at 40.degree. incidence than at normal due to the resonant frequency shift as the incidence angle changed from 0.degree. (normal) to 40.degree.. This resonant frequency shift was about 1.5 Gz. Thus, what is required is a flat FSS panel having a resonant frequency shift less than 1 GHz, particularly for the TM polarization, due to changes in the incidence angle from normal to about 40.degree. in any direction.